To improve fuel economy of vehicles, drivers are recommended, for example, to refrain from racing the engine, to stop idling the engine, and to avoid rapid acceleration. In addition to such operations that apparently lower fuel economy, fuel economy of a vehicle is also greatly influenced by how the accelerator is operated during driving. For this reason, display devices that display information for assisting a driver to improve the fuel economy are proposed (see, for example, patent documents 1 and 2).
Patent document 1 discloses a display device that assists a driver to drive a vehicle in a fuel-efficient manner. If the driver selects a fuel-economy shift mode, the disclosed display device determines whether the manner of driving of the driver is suitable to improve the fuel economy, and turns on an eco-lamp if the manner of driving is suitable to improve the fuel economy or turns off the eco-lamp if not.
Patent document 2 discloses a display unit that compares calculated current fuel economy with a recommended driving pattern provided for each type of vehicle and displays information indicating appropriateness of driver's accelerator operations (“appropriate”, “stop pressing”, and “too much”) in different colors to assist the driver to drive properly.
However, the display device disclosed in patent document 1 can provide only a small amount of information by turning on and off the eco-lamp according to the manner of driving of the driver. The eco-lamp only indicates whether the manner of driving is suitable to improve the fuel economy. Therefore, the driver cannot determine the degree he/she is allowed to depress the accelerator within a threshold for turning on or off the eco-lamp. As a result, a fuel-economy-conscious driver tends to drive the vehicle with a low accelerator level for a long period of time and may unintentionally lower the fuel economy. Also, when the driver operates the accelerator according to the eco-lamp, the driver tends to change the accelerator level frequently in the vicinity of the threshold for turning on or off the eco-lamp and may in turn lower the fuel economy.
The display device disclosed in patent document 2 calculates instantaneous fuel economy and compares the instantaneous fuel economy with fuel economy according to the recommended driving pattern. One problem is that since the instantaneous fuel economy is a value measured in the past, the display device cannot tell the driver how to operate the accelerator before the fuel economy decreases. Also, since the display device does not display causes of decrease in fuel economy, the driver cannot determine whether the decrease in fuel economy could have been prevented. Thus, with the display device of patent document 2, it is difficult to guide the driver to properly drive a vehicle to prevent decrease in fuel economy before it occurs. Also, as with the display device of patent document 1, the display device of patent document 2 does not tell the driver how much he/she is allowed to depress the accelerator before the accelerator level exceeds a threshold between “appropriate” and “too much”.
[Patent document 1] Japanese Patent Application Publication No. 2003-220851
[Patent document 2] Japanese Patent Application Publication No. 2002-370560